In the art of tissue making and papermaking in general, many additives have been proposed for specific purposes, such as increasing wet strength, improving softness, or control of wetting properties. For instance, in the past, wet strength agents have been added to paper products in order to increase the strength or otherwise control the properties of the product when contacted with water and/or when used in a wet environment. For example, wet strength agents are added to paper towels so that the paper towel may be used to wipe and scrub surfaces after being wetted without the towel disintegrating. Wet strength agents are also added to facial tissues to prevent the tissues from tearing when contacting fluids. In some applications, wet strength agents are also added to bath tissues to provide strength to the tissues during use. When added to bath tissues, however, the wet strength agents should not prevent the bath tissue from disintegrating when dropped in a commode and flushed into a sewer line. Wet strength agents added to bath tissues are sometimes referred to as temporary wet strength agents since they only maintain wet strength in the tissue for a specific length of time.
Typically, papermaking additives such as softeners, colorants, brighteners, strength agents, etc., are added to the fiber slurry upstream of the headbox in a papermaking machine during the manufacturing or converting stages of production to impart certain attributes to the finished product. These additives may be mixed in a stock chest or stock line where the fiber slurry has a fiber consistency of from between about 0.15 to about 5 percent or may be sprayed on to the wet or dry paper or tissue during production.
One difficulty associated with wet end additive addition is that the additives are suspended in water and must react with the cellulose through the relatively low numbers of carboxylic acid and/or aldehyde groups on the cellulose or alternatively the chemical must be cationic and attach via ionic attractions between itself and the anionic fiber. To improve absorption of wet end additives, the additives are often modified with functional groups to impart an electrical charge when in water. The electrokinetic attraction between charged additives and the anionically charged fiber surfaces aids in the deposition and retention of additives onto the fibers. Nevertheless, the amount of the additive that may be absorbed or retained in the paper machine wet end generally is limited to the number of anionic sites on the fiber and/or the number of reactive functionalities on the fiber. As a result, the absorption of additives may be significantly less than 100 percent, particularly when trying to achieve high additive loading levels.
Consequently, at any chemical level, and particularly at high addition levels, a fraction of the additive is retained on the fiber surface. The remaining fraction of the additive remains dissolved or dispersed in the suspending water phase. These unabsorbed or unretained additives may cause a number of problems in the papermaking process. The exact nature of the additive will determine the specific problems that may arise, but a partial list of problem that may result from unabsorbed or unretained additives includes: foam, deposits, contamination of other fiber streams, poor fiber retention on the machine, compromised chemical layer purity in multi-layer products, dissolved solids build-up in the water system, interactions with other process chemicals, felt or fabric plugging, excessive adhesion or release on dryer surfaces, and physical property variability in the finished product.
In addition, the amount of additive which may be retained on the fibers may be limited by the reactivity of the fiber, defined at least in part by the number of reactive sites on the fiber surface. As such, the desired characteristics of the paper product, such as wet and dry strength characteristics, for example, are also limited.
While cellulose fiber has been physically modified in the past through chemical treatment or enzyme treatment in order to increase the reactivity of the fiber with specific additives, these known treatments may be costly and difficult to control.
Therefore, what is lacking and needed in the art is a method for increasing the reactivity of a papermaking fiber and through that improving the physical characteristics of a web formed of the fibers due to the increased reactivity between the fibers and papermaking additives. In addition, increased reactivity of papermaking fibers may expand the number of possible additives which may be used in a papermaking process as well as minimize the excess additive in the suspending water phase due to increased absorption efficiency of the additive by the papermaking fibers.